Murine thymus-derived lymphocytes were sensitized in vitro to trinitrophenyl-(TNP-) modified autologous lymphocytes. Cytotoxic effector cells were generated which specifically lysed only Cr51-labelled spleen target cells modified with the same agent and shared H-2k and/or H-2D alleles with the cells of the sensitizing phase. Experiments using F1 hybrid responding lymphocytes sensitized and assayed with TNP-modified parental cells indicated that the homology required for lympholysis was either between modified stimulating and modified target cells, or among responding, modified stimulating and modified target cells. Antisera directed against serologically defined region products of unmodified target cells inhibited lysis of TNP-modified target cells, whereas anti-Ia antibodies had no detectable effect on lysis. Such experiments indicate that the effector cells are directed against specificities associated with serologically defined region products and that some TNP groups are sterically close to the H-2K and H-2D region antigens. Further immunogenetic studies indicated that cell-mediated lympholysis response potential to TNP-modified autologous components is controlled by H-2-linked Ir genes. These findings demonstrate bifunctional H-2-associated regulation of cellular immunity to modified autologous lymphocytes, and raise the possibility that a similar type of H-2-linked control of immunity may be relevant for surveillance against viral-associated neoplasia.